Request for reviews (M): 7017124: VM statistic use 32-bit values which may overflow
David.Holmes at oracle.com
Thu Feb 3 18:34:35 PST 2011
Vladimir Kozlov said the following on 02/04/11 12:17:
> David Holmes wrote:
>> Vladimir Kozlov said the following on 02/04/11 10:05:
>>> Added new Atomic method inc_counter() to increment long values,
>>> it is not precise on MP (lock is not used) but it is fine.
>> I strongly disagree! At least it has no place being in the Atomic::
>> set of functions as it is not atomic. It is not even monotonic! It is
> It is atomic since it loads and stores all 64 bits of julong atomically.
It is not atomic because it does not update the value atomically:
value += add_value;
> Why we can call atomic methods Atomic::load() and Atomic::store()
> which are used by new method but not this method?
Atomic::load and Atomic::store are required/expected to be atomic.
> And it is monotonic since value in memory only grows.
It should be monotonic but you need atomic updates for that to be true.
A counter can be updated by multiple threads simultaneously and that can
cause updates to appear out of order and hence the value can be seen to
>> extremely deceptive to do this. By all means change to julong where
>> needed, but unless they are truly going to be atomic updates (I
>> understand the overhead makes that prohibitive) then leave them as
>> simply x+=i; statements.
>>> Fixed several output formats to use VM FORMAT macros.
>>> I have to remove v9 check assert from Atomic_move_long() since
>>> it is called before VM_Version is initialized and C2 is built
>>> for v9 only anyway.
>>> Removed unused Chunk::clean_chunk_pool().
>> Used by CI.
>>> I fixed only statistic which was interesting to me.
>> So the CR is misleading. It purports to deal with 32-bit stat
>> overflows but in fact only deals with some stats, and primarily you
>> are using this to define PrintMallocStat - so the CR should reflect
>> that: "Fix some VM stats to avoid 32-bit overflow"
>> + "print malloc/free statictics") \
>> Typo: statictics
>> This change seems unrelated to the CR.
> I removed it from these changes.
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